1. Field of the Invention
The present invention relates to a self-healing passive optical network capable of detecting and healing cuts or deterioration of a feeder fiber or distribution fiber, thereby restoring the network by itself.
2. Description of the Related Art
A wavelength division multiplexing passive optical network (WDM-PON) can ensure the secrecy of communication and can easily accommodate special communication services required from each subscriber unit. The WDM-PON can enlarge channel capacity by assigning a specific wavelength to each subscriber unit and communicating with each subscriber unit. Also, the WDM-PON can easily increase the number of subscriber units by adding specific wavelengths to be assigned to new subscribers.
Generally, a WDM-PON uses a double star structure. That is, a central office (CO) and a remote node (RN) installed at an area adjacent to optical network units are connected to each other through one feeder fiber. The remote node and each optical network unit are connected to each other through a separate distribution fiber.
Multiplexed downstream optical signals are transmitted to the remote node through one feeder fiber. The multiplexed downstream optical signals are demultiplexed by a multiplexer/demultiplexer installed in the remote node and the demultiplexed signals are transmitted to subscriber units through the distribution fibers separately connected to optical network units.
Upstream signals outputted from the subscriber units are transmitted to the remote node through the distribution fibers separately connected to the optical network units. Then the multiplexer/demultiplexer installed in the remote node multiplexes the upstream signal according to each optical network unit, and transmits the multiplexed signal to the central office.
In the WDM-PON as described above, when an unexpected error occurs, such as a cut of a feeder fiber or a distribution fiber, a large quantity of transmitted data may be lost even though the error time period is short. For this reason, the error must be quickly detected and corrected.
Accordingly, it is necessary to develop a self-healing passive optical network (PON) capable of quickly detecting an error, such as a cut of a feeder fiber or a distribution fiber, on an installed optical link and correcting the error by itself.
FIGS. 1a and 1b are views of a conventional WDM self-healing ring network.
Generally, in a WDM optical communication network, a ring network connecting each node in a ring type is mainly used to smoothly cope with an error such as a cut of a transmission optical fiber.
The aforementioned conventional self-healing ring network connects a central office 100 to a first remote node 200 by means of two strands of optical fiber. Further, the self-healing ring network connects the central office 100 to a second remote node 300 by means of two strands of optical fiber.
Here, the two strands of optical fiber are a working fiber and a protection fiber. The central office 100 in a normal state transmits optical signals equal to each other, into which several wavelengths (e.g., λ1, λ2) of signals are multiplexed, through the two strands of optical fibers. The first remote node 200 or the second remote node 300 drops the optical signals inputted through the two strands of optical fiber to add/drop multiplexers 108 and 109 or add/drop multiplexers 112 and 113 and receives an optical signal having good characteristics from among the inputted optical signals by means of optical switching devices 110 and 111 or optical switching devices 114 and 115.
Meanwhile, the first remote node 200 or the second remote node 300 transmits optical signals equal to each other through the two strands of optical fiber. Then, the central office 100 demultiplexes optical signals according to each wavelength, and selects and receives one of two signals by means of optical switching devices 104 and 105.
FIG. 1b is a view illustrating a case in which an abnormality such as a cut of an optical fiber occurs in a working fiber.
When an abnormality occurs in the working fiber, the conventional self-healing ring network performs the following self-healing operation.
If the second remote node 300 cannot receive a second channel λ2 through the working fiber in a counterclockwise rotation it is assumed that the working fiber between the first remote node 200 and the second remote node 300 is cut., When it is assumed that the working fiber is cut, the second remote node 300 receives the second channel λ2 transmitted in a clockwise rotation through the protection fiber. Since the first remote node 200 cannot add and transmit a first channel λ1 through the working fiber in a counterclockwise rotation, the first remote node 200 switches the optical switching device 110 to transmit the first channel λ1 through the protection fiber in a clockwise rotation.
The aforementioned conventional self-healing ring network is efficient when a central office and a plurality of remote nodes are spaced away from each other by about several tens of kilometers. However, it is insufficient to introduce the aforementioned ring network structure to a PON which connects a central office to a remote node and connects the remote node to an optical network unit. That is, since a conventional PON has a star structure, a self-healing method having a concept different from a self-healing method in a ring network structure must be developed.
Furthermore, in the case of a WDM-PON using a wavelength injection method, an upstream/downstream injection light source exists and the directionality of the light source must be considered.